Calibration types of ports in ADS

"Single Mode port is excited using a calibration process that removes any undesired reactive effects of the port excitations at the port boundary.
Internal ports are not calibrated. The simulation with internal ports can take into account all of the EM coupling effects that will occur among ports in the circuit."
these are excerpted from ADS 2009 Documentation.

there are several points that I cannot get:

1. what's the so called "mode mismatch at the port boundary"?
2. What is the coupling effects among ports? Does it mean the coupling effect between different ports in the circuit? if it does, how to explain these coupling effect?

I have made some experiments with 2-port circuit using Momentum. And i find that the longer the length of every port (the signal path from - terminal to + terminal) and the further off the two ports, the less differece will show between internal port and single mode port. how to explain these?


thx